1. Field of the Invention
The invention relates to signal strength estimation in code division multiple access (CDMA) systems, and more particularly to signal strength estimation in CDMA systems.
2. Description of the Related Art
A conventional mobile communication system, such as a time division multiple access (TDMA) system or a frequency division multiple access (FDMA) system, is used to share a limited communication resource with multiple users. However, in response to growing demand for efficient use of frequency spectrum, attention is directed to a code division multiple access (CDMA) system as a multiplexing system. The CDMA system is a technique for simultaneously carrying out a plurality of communications by using signals in a same frequency band by means of the spread spectrum technique. Today, CDMA technique is very popular in wireless communication realized by various standard groups, especially in third generation communication systems such as 3GPP and 3GPP2. For a CDMA receiver, pilot channel signal sent from CDMA transmitter shall be acquired and correlated with corresponding traffic channels.
The signal to interference ratio (SIR) estimation is an important technique for CDMA receivers. In a CDMA receiver, many modem components utilize the estimated SIR result as a comparison threshold factor or as a decoding parameter, for components such as Turbo Decoder and Power control. Especially at power control, the SIR estimation error is closely related to the power control error and therefore is directly related to the service capacity. Better SIR estimation improves system performance, which relies on more precise interference and signal strength estimation.
In order to improve performance, pilot signal could be transmitted in space time diversity mode. In some cases, the pilot signals may be transmitted from more than one antenna in space time block coding based transmit diversity (STTD) mode. FIG. 1 is a block diagram of signal strength estimation circuit 100 of conventional CDMA systems by the use of dedicated physical control channel (DPCCH) pilot field. The signal strength estimation circuit 100 comprises a channel compensator 110, a space time transmit diversity decoder 120, a demodulator 130, and an average circuit 140. The channel compensator 110 compensates channel effect of received signal R. The STTD decoder 120 decodes the received signal R in a special case when the received signal R only has 2 pilot bits in a slot and comprises a first signal and a second signal obtained by STTD encoding the first signal, or directly outputs the received signal R otherwise. The demodulator 130 demodulates the received signal R by known pilot patterns. The average circuit 140 averages the received signal R to obtain the signal strength.
As an example, version 4.6 of 3GPP specification 25.211 could be used here to teach conventional application of STTD technique. A block diagram of a generic STTD encoder is shown in the FIG. 2. The STTD encoder operates on 4 symbols b0, b1, b2, b3 as shown in FIG. 2. In other words, it implies that STTD encoding shall be applied on a set of 4 symbols. There is no possibility to encode two symbols in STTD mode. Various physical channels could be transmitted in STTD mode, including various pilot channels.
FIG. 3 shows a special signal format transmitted from a CDMA transmitter. In FIG. 3, a first signal S1 transmitted by antenna 1 and a second signal S2 transmitted by antenna 2, and the second signal S2 is obtained by STTD encoding the first signal S1. In an example, the transmitted signal is mixed with data channel and pilot channel. The signal could be divided into frames, and each frame could be further divided into slots. In order to keep integrity of each slot, all symbols within a slot shall be modulated and encoded in the same way. It should be noted that 2 pilot bits of the second signal S2 are transmitted before the last 2 bits of data field, since STTD encoding needs to apply on 4 bits. Thus the receiver end cannot distinguish 2 pilot bits from the last 2 bits of data field in the special signal format without STTD decoding first because the last 2 bits of data field is unknown for the receiver end. Therefore, the STTD decoder 120 decodes the received signal R to get the 2 pilot bits only in a first iteration. Besides, the receiver end shall store this time slot and decode the data bits using decoded pilot bits later. Additional logic, memory space, computing resource, and time would be consumed to decode the data bits later. In this regards, the STTD decoder 120 applies only to the special signal format and STTD decoding is complicated, increasing costs and operation complexity.
Methods and apparatuses providing generalized signal strength estimation and preventing STTD decoding are thus desirable.